The present invention relates to a power switch having an electron emission type cold cathode.
The present invention also relates to a driving method of this kind of power switch.
In recent years, an electric field emission type cold cathode has greatly attracted attention because of its possibility to realize a high-speed response, its radiation resistance, and its possibility to accept a large current. Therefore, many developments have been made to this kind of cold cathode.
Those developments began with a proposal for a tunnel effect vacuum triode proposed by K. R. Shoulders in 1961 (ref. Microelectronics using electron-beam-activated machining techniques, Advances in Computers Vol. 1, p.p. 135 to 293).
In general, this field began to attract attention since a report prepared by SRI (Stanford Research Institute), which concerns a cold cathode ray tube using a thin film by C. A. Spindt (ref. Appl. Phys. 39, p. 3504, 1968).
This report discloses a method called a Spindt method for preparing a device and basis of its structure, with use of an oblique rotating evaporation method and a skilled means adopting sacrifice layer etching. The Spindt method is most widely used now. The device will be schematically explained with reference to FIGS. 1 to 3 showing a representative prior art example.
At first, as shown in FIG. 1, a thermal oxide film 62 is formed on an Si substrate 61, and for example, an Mo metal layer 63 as a metal layer for forming a control electrode is formed on the film 62. Further, the Mo metal layer 63 is subjected to patterning to form a control electrode opening portion 64, and thereafter, the thermal oxide film 62 is selectively etched.
Subsequently, as shown in FIG. 2, for example, an Al metal layer 65 is evaporated thinly as metal forming a sacrifice layer, and thereafter, for example, an Mo metal layer 66 as metal for forming a cold cathode is evaporated and formed by a oblique rotating evaporation method. Since evaporated metal sticks to the periphery of an opening portion, the opening portion is gradually reduced, and an Mo cold cathode 68 having a tapered tip like a cone is formed in the opening portion.
At last, as shown in FIG. 3, an excessive portion of the Mo metal layer 66 sticking to the control electrode 63 is removed together with the Al metal layer 65 as a sacrifice layer, and thus, a cold cathode ray tube is completed.
There are some other proposals than those described above, with respect to various specific structures. The basis of the element, however, is constructed by a cold cathode tapered end which easily generates electric field concentration, a control electrode provided in the vicinity of the cold cathode, and an electron capture electrode (anode) which receives electrons emitted.
Normally, a positive voltage is applied to the control electrode provided in the vicinity of the cold cathode, and a strong electric field of 10.sup.-7 V/cm or more is applied to the end of the cold cathode by a proximity effect and an electric field concentration effect at the end of the cold cathode. Further, electrons are emitted from the tapered end by a tunnel effect.
Emitted electrons are attracted to an anode by a positive voltage applied to the anode formed at a position opposite to the cold cathode, and a current flows between the cold cathode and the anode.
The current voltage characteristic of the element is schematically shown in FIG. 4. Here, it is important that the emission current from the cold cathode is determined by a voltage (which will hereinafter be simply called a control electrode voltage) applied between the control electrode and the cold cathode, and further, the amount of electrons which reach the anode among the emitted electrons changes, depending on the voltage (which will hereinafter be called an anode voltage) between the anode and the cold cathode.
This means, while a certain constant control electrode voltage is applied, the total number of electrons emitted from the cold cathode is constant, and all the electrons reach the anode only at the region where the anode voltage is sufficiently large, and the constant current value is attained. Suppose that this region is called a saturated region. In the saturated region, the current value increases and decreases only in accordance with the control electrode voltage.
Meanwhile, when the anode voltage decreases, the anode current decreases in accordance with a decreases in the anode voltage. This characteristic may seem to be just the static characteristic of an MOSFET. However, in case of a cold cathode ray tube, it should be noted that the number of electrons emitted from the cold cathode is not changed in this region, and excessive residual electrons flow to the control electrode as the anode current decreases. Broken lines in the figure are control electrode current values under conditions of respective control electrode voltages, the sum of the control electrode current and the anode current is substantially constant with respect to the control electrode voltage.
This kind of characteristic inherent to a conventional cold cathode ray tube does not cause problems with respect to such a use in which only the current to the anode is modified in case of a display, but causes a significant problem in case where the cold cathode ray tube is used as a switching element.
Specifically, where the cold cathode ray tube is to be used in a switching circuit which is applied to power device used for GTO,IGBT, etc., an element should originally have a characteristic of a simple switch, i.e., it is ideal that the element does not allow a current to flow even if a high voltage is applied while the switch is shut off, but the element allows a current to flow with an infinitesimal resistance drop in accordance with a voltage applied to the element from an external circuit.
This means, it is preferable that a current flows like a simple cable in accordance with Ohm's law and the internal resistance is as small as possible while the current capacity is as large as possible.
In case of a conventional cold cathode ray tube, however, as has been described above, the current value is essentially determined by a control electrode voltage applied. Specifically, in a saturated region, only a constant anode current flows through an element even when a voltage of an external circuit is changed, so that the element appears to behave like a current equalizer circuit having a variable internal resistance.
Meanwhile, in a non-saturated region, the anode current changes, depending on the voltage applied from an external circuit. Hence, the conventional cold cathode ray tube can be used as a switching element. However, the efficiency with which a current is used is extremely degraded since an excessive residual current flows through the control electrode.
For example, in the center of the non-saturated region, an half of all the electrons emitted from a cold cathode flows through the anode to the main circuit, and the power loss is 50% and is thus very large. Switching means control of transmission of an energy, itself, and it is a requisite that the element can be switched on and off with a low power loss.
As has been explained above, if a conventional cold cathode ray tube is regarded as a switching element, the cold cathode ray tube causes a problem that a constant current is caused with a control electrode voltage without depending on the voltage applied from an external circuit or a problem that only a low efficiency is attained due to a loss current to the control electrode. It is thus difficult to use a conventional cold cathode ray tube as a switching element.